70 THE IRBIGA 2 ION A OE . 



strength. In such cases it is safer to make the dam of timber or 

 masonry arching upstream, bedded well into the banks, and puddled 

 well above with clay, to prevent leakage, which would soon under- 

 mine and carry down an otherwise good structure. Such a dam will 

 cause the bed of the stream above it to fill with sediment, which will 

 reduce the direct pressure. The beginner, however, will do well to 

 err on the safe side, if at all, and make his dam twice as strong as 

 might seem on first thought to be necessary, 



In a wider stream in a shallow bed enough water can often be 

 diverted by a wing dam, starting just below the head of the ditch and 

 running obliquely up the stream toward its center. Such a dam can 

 be easily constructed of posts and brush, or of any coarse heavy 

 material which is ready at hand or most cheaply secured. It raises 

 part of the water sufficiently for outflow into a ditch aud is not enough 

 of an obstruction to be torn out in high water, or, if the dam is partly 

 destroyed, it can be cheaply replaced. 



At the point of diversion from the stream a head gate should be 

 put in. This is a very simple construction of plank with a sliding 

 gate capable of being raised and lowered. It wiJl protect the ditch 

 by keeping the water out in time of high water. It should have a 

 cross plank on the stream end, so that the water shall fall over the 

 plank, as this will pi event much heavy sediment from entering the 

 ditch. Obviously the dam must raise the water sufficiently to sur- 

 mount this obstruction. The accompanying drawing and description 

 of a small head gate are taken from the Yearbook of the United States 

 Department of Agriculture for 1900. 



Fig. 5 shows a common type of small headgates. It consists of a box or flume 6 

 feet long, 3 feet wide, and 3 feet deep, with a gate (D) at the end nearest the creek. 

 At both ends the sides flare at an angle of about 30 degrees. Under them 1? feet 

 below the floor of the structure (C) platforms (A and B) are built- Both of these 

 platforms are covered with earth to the level of the floor (C). Earth is also care- 

 fully tamped around the outside of the headgate. 



All precautions should be taken to prevent water from working along the outside 

 of the headgate. The structure may be undermined in a short time if only a small 

 stream finds its way between the planks and the earth. The flaring wings and sub- 

 merged platform are built to prevent this action and also to make the structure se- 

 cure in case of high water. 



(To be continued.) 



